Due to the rapidly rising price of fossil fuels and a growing desire to reduce the environmental impact of non-renewable fuels, ethanol has become a significant part of the transportation fuel mix. Ethanol made by fermentation of plant derived starches and sugars is considered to have a lower environmental impact than fossil fuels.
Ethanol is usually produced from starch or sugar by fermentation. In North America the feedstock is primarily corn. There are disadvantages to using potential food or feed plants to produce fuel. Moreover, the availability of such feedstock is limited by the overall available area of suitable agricultural land.
To reduce the amount of food or feed plants in ethanol production, many alternate feedstocks have been proposed, among them lignocellulosic biomass. This includes cellulose containing agricultural and wood residues, purpose grown non-food crops, and a wide variety of biodegradable wastes.
Agricultural and wood residues and non-food crops have several economic and environmental advantages over corn and starch. Furthermore, some alternative crops such as Miscanthus, Switchgrass and hybrid Poplar can even grow on poor quality land not suitable for corn. Wood and agricultural residues have relatively low market value and have the potential to be high volume renewable feedstocks for ethanol production.
Lignocellulosic biomass is composed of three major polymers: cellulose, hemicellulose and lignin. Cellulose makes up 40% to 60% of lignocellulosic biomass and is the desired target for ethanol production. Cellulose resembles starch in many ways. It is a homogeneous polymer made of linked glucose monomers, as is starch. Cellulose, however, is much more difficult to depolymerize than starch. This is due to a difference in the nature of the glucose linkages as well as the presence of hemicellulose and lignin. As a result, more severe conditions are needed to hydrolyze cellulose to glucose than are needed to hydrolyze starch.
The challenge in the production of fuels from biomass is to remove the non-cellulosic components of the biomass to make subsequent treatment easier at lower capital and operating costs.
One process for converting lignocellulosics to ethanol can be called the enzymatic hydrolysis process. This process requires five major unit operations: feed preparation, pretreatment, enzymatic hydrolysis, fermentation and distillation. Lignocellulosic biomass is chopped, cleaned, and ground to the desired size.
Pretreatment of the biomass opens up its structure, exposing the cellulose to the hydrolytic action of enzymes in the hydrolysis step. Pretreatment also increases the concentration of cellulose in the prehydrolysate, which improves the digestibility of the cellulose by enzymes.
In the enzymatic hydrolysis step, the prehydrolysate obtained in the pretreatment step is cooled to about 40° C. to 60° C., cellulase enzymes are added and the hydrolysis is allowed to continue to achieve the desired conversion of cellulose to glucose. Fermentation of the sugars in the hydrolysate by yeast is the next step.
In the final step, ethanol is recovered by distillation of the fermented mash and dehydration of ethanol to the desired concentration. Many different configurations for this step are practiced in the industry.
Lignocellulosic biomass contains a variety of chemicals and polymers which reduce access to the cellulose molecule.
Lignin is a potent inhibitor of hydrolysis and some soluble lignin derivatives inhibit the fermentation process. Thus, it is desirable to use a lignocellulosic feedstock which is low in lignin. The lignin content of corncobs, (less than 8% by weight) is low, which would make this a good biomass feedstock for the production of ethanol. However the hemicellulose content of corncobs is high, almost 30% of the total dry matter. Moreover, much of the hemicellulose is acetylated. The dissolution of hemicellulose leads to the formation of acetic acid, a powerful inhibitor of the yeast fermentation process used to produce ethanol. This is a problem, since the acid remains in the pretreated biomass and carries through to the hydrolysis and fermentation steps.
Many of the compounds released in the pretreatment step, such as acetic acid, hemicellulose and many hemicellulose degradation products are also inhibitors of and retard the downstream fermentation process. This results in increased capital equipment costs for removal of the inhibitory compounds and frequently incomplete conversion of the glucose to ethanol. Therefore, it would be desirable to remove these inhibitory compounds prior to the enzymatic hydrolysis step.
In known pretreatment processes, acidic solutions, for example mineral acids such as sulfuric acid, or alkaline solutions are added to the biomass for hydrolysis of the biomass components. This chemical treatment is disadvantageous since large amounts of water are required to flush the treatment chemicals from the pretreated cellulose prior to the enzymatic hydrolysis and fermentation steps.
In an alternate approach, a steam gun cellulose pretreatment is used. Biomass ground to the desired size is subjected to steam under pressure and at elevated temperatures. The pressure is then released rapidly by way of a fast acting valve, leading to an explosion of the cooked biomass material. This process does not require the addition of chemical processing agents, but depending on the process conditions will produce significant amounts of undesirable by-products which are detrimental to the downstream hydrolysis and fermentation steps.
It is a challenge of the enzymatic hydrolysis process to operate the process at the most optimal conditions for the respective feedstock used. Strong pretreatment conditions would appear desirable to unlock as much of the cellulose as possible and hydrolyze as much of the hemicellulose as possible. Aggressive pretreatment conditions (higher heat for longer duration) release more acetic acid especially when feedstocks with high acetylated hemicellulose content are used, such as corncob. This elevated acetic acid content accelerates the hemicellulose breakdown by autohydrolysis. However, an elevated acetic acid content significantly retards the fermentation of glucose to ethanol and may even result in incomplete fermentation. Other hemicellulose degradation products generated also inhibit fermentation and may even interfere with cellulose hydrolysis by crystallizing on and effectively capping the cellulose fibers, making them inaccessible to enzymatic hydrolysis. Thus, a process is desired which removes an amount of inhibitory compounds in the pretreated cellulose stream, in order to enhance the digestibility of the cellulose in the enzymatic hydrolysis and improve the efficiency of the conversion of glucose to ethanol in the fermentation step.